Dashpot timer having a cocked piston

ABSTRACT

A TIMING MECHANISM OF THE DASHPOT TYPE WHEREIN A CYLINDRICAL PISTON TRAVELS IN A CYLINDER AT A CONTROLLED RATE. THE AXIS OF THE PISTON IS COCKED RELATIVE TO THE AXIS OF THE CYLINDER SO AS TO INCREASE THE TIME REQUIRED FOR THE PISTON TO TRAVEL LONGITUDINALLY THROUGH THE CYLINDER AND TO IMPROVE THE CONSISTENCY OF THAT TIME. MEANS ARE PRO-   VIDED TO MAINTAIN THE AXIS OF THE PISTON COCKED RELATIVE TO THAT OF THE CYLINDER.

Jan. 12,1973 o. s. BREED 3,553,954

DASHPOT TIMER HAVING A COOKED PISTON ori inal Filed April 1, 1969 0E TKA VEL INVENTOR DAV/D 5. aft-ED m Z/J QMW ATTORNEYS United States Patent O rm. (:1. G04f US. Cl. 58-1 7 Claims ABSTRACT OF THE DISCLOSURE A timing mechanism of the dashpot type wherein a cylindrical piston travels in a cylinder at a controlled rate. The axis of the piston is cocked relative to the axis of the cylinder so as to increase the time required for the piston to travel longitudinally through the cylinder and to improve the consistency of that time. Means are provided to maintain the axis of the piston cocked relative to that of the cylinder.

CROSS-REFERENCE TO OTHER APPLICATIONS This application is a division of application Ser. No. 811,968, filed Apr. 1, 1969. The present application relates to a timer comprising a piston which travels in a cylinder at a controlled rate due to the predictable flow of a fluid contained in the cylinder through a predetermined clearance between the piston and interior cylinder Wall to occupy the ever-increasing volume behind the piston. Dashpot timers of this general type are further described in US. Pat. No. 3,171,245, issued on Mar. 2, 1965, and the currently pending application Ser. No. 687,207, filed Dec. 1, 1967, now US. Pat. No. 3,458,991.

BACKGROUND OF THE INVENTION In the above cited references, a dashpot timer comprising a cylindrical piston disposed within a cylinder filled with a fluid in either gaseous or liquid form, is disclosed. In order to obtain consistency and predictability in the rate of travel of the piston through the cylinder over several runs, it has been proposed to dispose the piston against the wall of the cylinder by various means, such as by inclining or tilting the cylinder. Also, it has been suggested that coating the surface of the piston with a polymeric material, such as Teflon, contributes to the predictability of the fluid flow rate between the piston and cylinder wall and hence the rate of descent by maintaining the piston in the center of the cylinder.

In many instances, particularly in military and aerospace applications, space limitations require that the piston of such a dashpot timer accurately travel a relatively small distance through its associated cylinder over an extended period of time and, of course, that such travel be consistently accurate over several runs and from unit to unit. Although this has been obtained to a great extent by coating the dashpot timer piston in the manner described in the pending applications so as to keep the piston substantially in the center of the cylinder, there are still many applications, particularly in connection with military ordnance devices where that solution is not preferred and/r slower piston travel rate is required. In this connection, it should be noted that maintaining the piston in the center of the cylinder increases the travel time by a factor of about 2.5 (over keeping the piston against the wall of the cylinder) whereas in accordance with the present invention, factors in excess of are dramatically available.

It is, therefore, the principal object of the present invention to provide an accurate dashpot timer for use in those applications where space or other limitations are prime considerations for insuring the consistent positioning of the piston within the cylinder and to provide such a timer which is exceptionally reliable, susceptible to long life, relatively inexpensive to manufacture, and which is accurate over a wide range of temperatures.

SUMMARY OF THE INVENTION These and other beneficial objects and advantages are attained in accordance with the present invention by providing a dashpot timer comprising a cylinder having a substantially cylindrical interior wall with a piston disposed for movement within the cylinder and having an outer diameter slightly less than the inner diameter of the cylinder whereby to define a substantially annular orifice between the piston and cylinder. The timer is further provided with means for cocking the longitudinal axis of the piston relative to the longitudinal axis of the cylinder and for maintaining the piston in a cocked position throughout the travel of the piston through the cylinder. The motivation behind the use of a piston, the orientation of which is other than parallel to the axis of the associated cylinder is that by so doing, a circumferential flow of fluid is created within the cylinder as displaced fluids are shifted to a position behind the piston as the piston descends through the cylinder. This circumferential flow causes a slow down in the rate at which fluid is displaced and hence the rate at which the piston can descend. The circumferential flow also causes hydrodynamic forces to be built up within the cylinder which tend to align the piston and to shift it to the center of the cylinder. These hydrodynamic forces must be overcome by the cocking means which thus must remain in operative contact with the piston throughout the pistons descent.

Other objects and advantages of the present invention will become apparent to those skilled in the art from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings:

FIG. 1 is a fragmentary, side elevational, partial sectional view of a dashpot timer incorporating the teachings of the present invention;

FIG. 2 is a horizontal sectional view thereof taken along reference line 22 of FIG. 1 in the direction indicated by the arrows; and

FIG. 3 is a graph which shows the effect on time which the piston position has as the piston is moved from side travel to cocked travel for a piston whose length to diameter ratio is .5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The present invention is depicted in the accompanying drawings wherein similar components bear the same reference numeral throughout the several views. Reference is now made to FIGS. 1 and 2 wherein a dashpot timer 10 is illustrated which incorporates the teachings of the present invention. The timer 10 includes a cylinder 12 and a piston 14 slidably disposed therein. The dimensional tolerance of the cylindrical interior wall 16 of the cylinder and the cylindrical exterior wall 18 of the piston 14 provide for an annular orifice 20 through which a fluid, in gaseous form, is adapted to flow. The mating diameters represented by the respective walls 16 and 18 are very closely dimensionally controlled to provide a known clearance (in the form of orifice 20) through which a known amount of gas must pass from the begin ning to end of the prescribed time interval (represented by the length of longitudinal travel of the piston 14 in the cylinder). In this connection, at least one of the cylinder ends 17 and 19 is sealed to assure fluid flow through the orifice 20. Certain applications will provide for the sealing of either or both cylinder ends. In the disclosed embodiment of FIG. 1, end 19 of cylinder 12 is closed while end 17 is open. Further, one or both of the walls 16 and 18 may be coated with a polymeric, friction resistant coating such as Teflon, as described in the pending application.

The piston, which is cylindrical in shape, extends between terminal shoulders 22 and 24. An axially aligned extension 26 of the piston extends outwardly beyond one end of the cylinder from shoulder 24 and a spring 28 is provided bearing on the extension 26 and extending transverse thereto. The spring 28 has one end 30 fixed with respect to the cylinder and the other end 32 in contact with the extension 26 thereby exerting a transverse force on the piston causing a slight rotation of the longitudinal axis of the piston relative to the cylinder axis so as to cock the piston relative to the cylinder axis. Cocking the piston in the manner described effectively increases the path that fluid displaced from in front of the piston must travel by causing the fluid to assume a circumferential path, as shown by the arrows on FIG. 1, rather than a direct longitudinal path, and hence increases the time required for the piston to travel a given longitudinal distance along the cylinder axis. This circumferential fluid flow causes hydrodynamic forces to be built up which tend to align the axis of the piston with that of the cylinder. These forces are countered by the biasing force of spring 28 which thus must continuously contact the piston and exert a transverse force thereon to maintain the piston in a cocked position throughout its travel path.

It has been noted experimentally and through a computer analysis of the operation of the present dashpot timer that the time required for the piston to travel a predetermined distance within the cylinder varies in the manner illustrated in the graph of FIG. 3, with the angle which the side of the piston makes with the side of the cylinder. The angle 0 will, in turn, be determined by the length of the piston and the clearance between the piston and cylinder. Thus, in the referenced graph, e represents the percentage of cocked travel; that is, when e=l, the piston is fully cocked to the position shown in FIG. 1 and when e:0 the piston is resting against the side of the cylinder with side Wall 18 of the piston abutting side wall 16 of the cylinder. The important point which this graph demonstrates is that when the piston is in a cocked position, the edge of the piston becomes extremely important. If that edge wears slightly, or if it is not uniform around the circumference of the piston, it may cause a slight change in the value of e and hence, may make a significant change in the time delay. Thus, a minute abrasion of the Teflon coat on the piston edge could effect the amount which the piston is cocked, and since the time curve is very nearly vertical as e approaches 1 and the piston approaches a fully cocked position, a slight change in the degree of cocking will have a significant change in the time delay. Consequently, care must be taken to assure that piston edges do not contain any sharp corners where the wear rate would be high.

In practice, it has been found that if the piston edges are beveled through a careful barrel finishing operation, a gradual curve 32 blending into a radius of about to thousandths of an inch can be formed on the top and bottom peripheral edges of the piston, this effect can be essentially eliminated and the advantages of the increased time achieved by cocked travel are obtained. The curved ends are shown in greatly exaggerated form in FIG. 1.

With regard to the above, it has also been noted that if the cocking force, that is, the force exerted on the piston extension 26 by spring 28 is excessive, this could cause a high wear rate of the Teflon coating. The cocking force should, therefore, be just enough to overcome the hydrodynamic forces developed with some degree of safety and no greater. Anything in excess of this amount will create inconsistency problems due to abrasion of the Teflon film and to relatively high frictional forces between the piston edges and the cylinder wall.

' Thus, in accordance with the above, the several aforementioned objects are eflectively attained.

I claim:

1. A dashpot timing device comprising a cylinder having a substantially cylindrical interior wall; a piston disposed for movement within said cylinder having an outer diameter slightly less than the diameter of said interior wall whereby a substantially annular orifice is defined between the piston and cylinder; and means for cocking the longitudinal axis of said piston relative to the longitudinal axis of said cylinder and for maintaining said piston in a cocked position throughout the travel of said piston through said cylinder.

2. The invention in accordance with claim 1 wherein said piston comprises a cylindrical body portion disposed within said cylinder and further comprising an extension of said body portion extending outwardly from one end thereof beyond an open end of said cylinder wherein said cocking means includes a spring extending transverse to the longitudinal axis of said cylinder a agvhnrnidqfi the longitudinal axis of said cylinder and having one end fixed relative to said cylinder and the other end adapted to exert a transverse biasing force on said piston extension whereby to cause rotation of the longitudinal axis of said piston relative to the longitudinal axis of said cylinder.

3. The invention in accordance with claim 2 wherein said piston extension is in axial alignment with the longitudinal axis of said cylindrical body portion.

4. The invention in accordance with claim 2 wherein the inner surface of said cylinder and/or the outer surface of said piston are coated with a friction resisting polymeric coating material.

5. The invention in accordance with claim 4 wherein said polymeric coating material is Teflon.

6. The invention in accordance with claim 2 wherein the outer peripheral top and bottom edges of said piston are provided with a gradual curve ending in a radius.

7. The invention in accordance with claim 6 wherein said radius is from 5 to 15 thousandths of an inch.

References Cited UNITED STATES PATENTS 2,417,641 3/1947 Fischer 58-1 RICHARD B. WILKINSON, Primary Examiner E. C. SIMMONS, Assistant Examiner US. Cl. X.R. 58144 

